1. Technical Field
The present invention relates to a system for controlling the rotating speed of a spindle driven by pneumatic power and more specifically it relates to such a system capable of maintaining the rotating speed of the spindle constant independently of a load imposed on the spindle.
2. Background Art
A machine tool, as illustrated in FIG. 1 of the accompanying drawings, which employs a pneumatically driven spindle has been previously developed to improve cutting surface accuracy as well as cutting efficiency. A spindle 2 is rotatably disposed in a spindle casing 1. Mounted on one end of the spindle 2 is a tool 4 for cutting a workpiece 3, and mounted on the other end thereof is an air turbine 5 for receiving air so as to rotate the spindle 2. The air turbine 5 is driven by air 6 of constant flow rate which is supplied from a source (not shown).
Two types of non-contact, rotating speed detecting arrangements are generally known for use with the above mentioned spindle. One is an arrangement associated with an optical sensor, wherein light deflected by the spindle is received by the optical sensor and the rotating speed is calculated by peripheral elements. The shortcoming of this arrangement is that the detecting accuracy of the sensor is deteriorated when lubrication oil, for example, adheres to the spindle.
FIG. 2 illustrates the other type of non-contact, rotating speed detecting arrangement. The air turbine 5 is fixed on the spindle 2 by a nut 10, and a gear 7 is also attached to the spindle 2. The gear 7 is a spur gear, for instance, and it possesses a convexo-concave part 8 along the periphery thereof. A rotating speed detector in the form of an electromagnetic pickup 9 is provided near the gear 7 in the radial direction of the gear 7. The magnetic power between the convexo-concave part 8 and the electromagnetic pickup 9 varies as the spindle 2 rotates, and electrical pulses are produced due to the changing magnetic power, which are in turn processed to yield the rotating speed. In this arrangement, space for the pickup 9 is required in the radial direction of the gear 7. This means that the dimensions of the spindle casing 1 of FIG. 1 have to be large. Also, the rotating speed of the spindle 2 changes due to the cutting load since the flow rate of the air 6 is constant. More specifically, as shown in FIG. 3, the actual rotating speed of the spindle, N1 is set to the desired rotating speed N during mode A. During the mode B in which the tool 4 is contacting the work 3, the actual rotating speed N1 is decreased by the cutting load until the rotating energy and the braking effort are in balance with each other. After that, the rotating speed of the spindle 2, N1 is raised to the desired value N by an operator by changing the flow rate of the air supplied to the turbine 5. Upon the completion of cutting, the spindle 2 leaves the workpiece 3 and at this time the actual rotating speed N1 unnecessarily increases (mode C). It is also predicted that the manual speed control during the mode B will result in non-preferred cutting. Furthermore, in the event that the depth of cutting is program-controlled and an unduly large depth of cutting is programmed, the spindle 2 would be stopped when the resistance force against the spindle overcomes the rotating torque of the spindle 2. In this case, the spindle 2 starts rotating at a very high speed as it leaves the workpiece 3, which is dangerous.